The ohmic contact electrode provides an important medium and path for the operational current required by the light-emitting device. However, the currently available light-emitting devices require a higher annealing temperature and a more complicated manufacturing procedure to form an electrode with a good ohmic contact property. Therefore, how to improve the property of the ohmic contact electrode and simplify the manufacturing procedure to reduce the cost becomes the important subject of the present research. According to the prior art, the manufacturing methods for the ohmic contact electrode of the light-emitting devices are mainly divided into a two-step manufacturing procedure and a single-step manufacturing procedure.
With respect to the two-step manufacturing procedure, appropriate metallic films are formed on the epitaxial structure on the bottom surface and the upper surface of a substrate (gallium arsenide, for instance) at first, such as gold-germanium alloy/nickel/gold on the N-type gallium arsenide (GaAs) substrate or gold-zinc alloy on the P-type GaAs substrate by a vapor phase deposition process. The metallic films form the ohmic contact electrode with the ohmic contact property through an annealing process. Finally, a thick metallic pad (for example a thick gold layer) is formed on the ohmic contact electrode for the subsequent metal wire bonding. This method requires a high annealing temperature (higher than 380° C.), and the surface of the ohmic contact electrode after the annealing process is very rough and inadequate for the subsequent metal wire bonding. Additionally, at least two lithographic processes are required, so the manufacturing procedure is complicated.
Regarding the single-step manufacturing procedure, the above-mentioned thick metallic pad and the ohmic contact metallic films are formed in the same manufacturing procedure, and an annealing process is then performed to form the ohmic contact electrode. For example, after forming the gold-germanium alloy/nickel/gold on the N-type GaAs substrate and titanium/palladium/gold alloy on the P-type GaAs substrate by the vapor phase deposition, the ohmic contact electrode and the thick metallic pad are then formed by an annealing process simultaneously. The advantage of the single-step manufacturing procedure is the simpler process, and that the surface 6f the ohmic contact electrode still stays uniform after the annealing process is performed, which is adequate for the subsequent metal wire bonding.
The P-type ohmic contact electrode formed on the P-type GaAs usually consists of multiple metallic layers. For example, German patent (DE Patent No. 4,401,858) discloses a P-type ohmic contact electrode consisting of a plurality of gold/gold-zinc/gold layers by the vapor phase deposition, wherein the annealing temperature is between 360° C. and 480° C. In addition, U.S. Pat. No. 5,523,623 discloses a P-type ohmic contact electrode consisting of a plurality of nickel/titanium/platinum layers or nickel/titanium/palladium by the vapor phase deposition, wherein the annealing temperature is between 370° C. and 420° C.
FIG. 1 is a cross-sectional diagram of a light-emitting device 10 according to the prior art. As shown in FIG. 1, the light-emitting device 10 includes a substrate 12, an epitaxial structure 20 positioned on the upper surface of the substrate 12, an N-type ohmic contact electrode 14 positioned on the bottom surface of the substrate 12, and a P-type ohmic contact electrode 30 positioned on the epitaxial structure 20. The epitaxial structure 20 consists of a first reflection layer 22, a light-emitting layer 24, a second reflection layer 26 and a P-type contact layer 16, wherein the reflectivity of the first reflection layer 22 is larger than or equal to that of the second reflection layer 26.
The P-type ohmic contact electrode 30 consists of gold-beryllium alloy or gold-zinc alloy. When the gold-beryllium alloy or the gold-zinc alloy is used to form the P-type ohmic contact electrode 30, an annealing process must be performed at a high temperature between 380° C. and 400° C. to ensure the formation of a preferred ohmic contact electrode. However, the annealing process at the high temperature between 380° C. and 400° C. will result in deterioration of the surface of the P-type ohmic contact electrode 30, which is inadequate for the subsequent metal wire bonding. On the contrary, to ensure the formation of the P-type contact electrode 30 with a surface appropriate for the subsequent metal wire bonding, the temperature of the annealing process has to be decreased, which however will result in deterioration of the ohmic contact property between the P-type ohmic contact electrode 30 and the P-type contact layer 16.
FIG. 2 is a cross-sectional diagram of another light-emitting device 40 according to the prior. The difference between FIG. 2 and FIG. 1 is that the P-type ohmic contact electrode 30 in FIG. 2 is a double-layer structure. As shown in FIG. 2, the P-type ohmic contact electrode 30 consists of a nickel layer 31 and a gold layer 32. The nickel layer 31 and the P-type contact layer 16 can form good adhesion characteristics, while the gold layer 32 is appropriate for the subsequent metal wire bonding. In order to ensure the formation of an ideal ohmic contact property with the P-type contact layer 16, an annealing process must be performed at approximately 400° C. to the P-type ohmic contact electrode 30 consisting of the nickel layer 31 and the gold layer 32.
FIG. 3 shows the surface of the P-type ohmic contact electrode 30 in FIG. 2 after the annealing process is performed at 400° C. for sixty seconds (magnification ratio: 400 times). The circular area is the epitaxial layer that does not require to deposit the P-type contact electrode metal, and the thickness of the nickel layer 31 is 50 nanometer (nm) and the thickness of gold layer 32 is 350 nm. Obviously, the surface of P-type ohmic contact electrode 30 has many concaves and convexes, which is inadequate for the subsequent metal wire bonding.
FIG. 4 is a cross-sectional diagram of another light-emitting device 50 according to the prior art. FIG. 4 is different from FIG. 1 and FIG. 2 in that the P-type ohmic contact electrode 30 is a three-layer structure. As shown in FIG. 4, the P-type ohmic contact electrode 30 consists of a titanium layer 33, a platinum layer 34 and a gold layer 35. The annealing temperature can be decreased to 350° C., and the P-type ohmic contact electrode 30 still possesses an appropriate surface for the subsequent metal wire bonding.
FIG. 5 shows the input current/output power curve of the light-emitting device 50 in FIG. 4. As shown in FIG. 5, the input current/output power curve of the light-emitting device 50 is not linear (the slope of the curve decreases with the increase of the input current), which makes it difficult to control the output power of the light-emitting device 50 by the input current.
From the above-mentioned prior art, the formation of the P-type ohmic contact electrode requires a higher annealing temperature, which will result in the unwanted cross-diffusion between layers of the epitaxial structure and damage the optoelectrical property of the device. Therefore, how to reduce the annealing temperature and form the ideal ohmic contact electrode becomes an important subject matter.